The yeast, Saccharomyces cerevisiae is a facultative aerobe that regulates the expression of a large number of genes in response to the availability of molecular oxygen. There are three broad categories of nuclear-encoded, oxygen-regulated genes. One set, encoding respiratory and oxidative stress functions, is expressed at high oxygen levels. A second set, the hypoxic genes, is expressed at low oxygen concentrations. This set encodes functions also involved in respiration and the biosynthesis of heme, sterols, and fatty acids which require oxygen as an electron acceptor in specific enzymatic reactions. This set increases the efficiency of electron transport and these biosynthetic reactions when oxygen becomes limiting. The last set represents functions required for true anaerobiosis. For the first two sets of genes, heme levels serve as the cellular signal for oxygen availability; heme biosynthesis requires oxygen and its levels fall as oxygen becomes limiting. Thus the respiratory and oxidative stress functions are heme-activated, while the hypoxic functions are heme-repressed. The goal of the proposed research is to elucidate the molecular mechanisms of heme regulation. Heme regulation is exerted at the level of transcription. Heme-activated genes are regulated by two transcriptional activators, HAP1 or HAP2/3/4, while heme repression is mediated by the repressor protein ROX1. The proposed studies will focus on the nature of a possible HAP1 and ROX1 interaction, and the mechanism of ROX1 repression. Repression by ROX1 requires a set of additional proteins that are also required for repression in several unrelated pathways, suggesting a general mechanism for transcriptional repression. The regulation of genes by heme represents the coordination of a number metabolic pathways, many of which are additional regulated by more specific mechanisms. The complexity of these systems suggest that it will serve as an excellent model for the coordination of metabolism in higher organisms as well as a more specific model for the response of cells to changes in oxygen levels and oxidative stress.